This invention relates to the casting of molten metal and in particular to vessels for molten solids, e.g. molten metal, and to outlets for said vessels.
Problems occur when molten solids are poured through an outlet from a vessel, for example when pouring molten metal. In a foundry, it is known to fit an outlet through the floor of a vessel for molten metal, e.g. a foundry ladle, and to provide a so-called stopper rod above the vessel. The rod is lowered or raised to close or open the outlet. The rod comprises a metal shaft surrounded by a refractory sleeve. This arrangement has disadvantages. For example, the rod becomes distorted in use; if it is used as a throttle, the stream of metal breaks up affecting the quality of the casting; and there is a relatively long turn around period when changing the refractory sleeve of the stopper rod.
An alternative device is a so-called sliding gate valve. This is mounted externally below the bottom outlet and is used on bulk steel ladles where the liquid metal is discharged in a few pours. In the foundry, however, the outlet is opened and closed a greater number of times and there can be long intervals between one opening and the next. When the sliding gate valve is closed, a slug of metal forms in the outlet above the closed valve and is unable to circulate, even though it is communication with the bulk of the molten metal, and the slug tends to freeze if the time between successive pours is too great. This causes metal to freeze on the sliding gate valve which is thus not suitable for use with foundry ladles. The sliding gate valve is also not without problems in relation to liquid steel which is particularly prone to this "freezing-off" and there is thus a serious risk of valve blockage.
It has been realised that there should be advantages in having a rotary valve mechanism instead of a stopper rod system or a sliding gate valve system. The mechanism comprises a valve in a seating therefor in the vessel outlet. A passage is present in the valve member which has inlets in the head portion thereof. A drive mechanism is present outside the vessel and used to rotate the valve member with respect to the seating member therefor, the seating member being shaped so that when in one position the inlet is closed off and in another open. Such a system is disclosed in WO88/05355 published in July 1988; in GB-A-2226263 published in June 1990; and in WO 90/14907 published in December 1990. More specifically, GB-A-2226263 discloses an outlet comprising a rotary valve member and a seating therefor, the valve member having at least one passage for conducting molten solid from inside to outside the vessel, the valve member sealing with a wall of the seating member, the wall having a first section sufficiently high that when the valve member is rotated to align an inlet end of the passage therewith the passage is closed by the first section, the wall having a second section of less height so that if the valve member is rotated to align the inlet end of the passage therewith, the inlet end is open. The outlet is normally installed so that the top of the second section of the wall is level with the inside surface of the floor of the vessel. The first section does not extend round the periphery of the wall significantly further than necessary to close the outlet. The majority of the valve member is therefore exposed so that, in contrast to the conditions when a sliding gate valve is closed, the molten metal is able to circulate around the closed valve member and there is no slug of trapped metal liable to freeze between pours.
In each of these prior disclosures, insufficient attention has been paid to the practical problems of operating such a rotary valve mechanism. It has now been discovered that in order to reliably and repeatedly rotate the valve member it is necessary to shape the sealing faces as described herein and arrange the valve so that there are no dead zones wherein molten solid can be trapped; and preferably also shape the exit portion of the passage so that the stream of molten solid emerges in a predetermined shade.
In one aspect the invention provides an outlet for a vessel containing molten solid, the outlet comprising a rotary valve member and a seating member therefor, the valve member having a passage to conduct molten solid from inside to outside the vessel, the valve member sealing with a wall of the seating member, the wall having a first section sufficiently high so that when the valve member is rotated to align an inlet end of the passage therewith the inlet end is closed by the first section, the wall having a second section of relatively reduced height so that when the valve member is rotated to align an inlet end of the passage therewith the inlet end is open to allow molten solid to pass into and along the passage characterised in that the first section of the wall provides upstanding side walls for the second section whereby the second section provides the base of a generally open channel cross-sectional shape so that molten solid can pass freely in the region of the second section when the inlet is closed, and in that the opposite surfaces of the valve member and the wall of the sealing member which mate are convex in a plane including the axis of rotation of the valve member to reduce sticking in use.
Thus, because the first section provides upstanding side walls for the second section which defines the base of a channel of generally open cross-sectional shape, i.e. no roof is present, there is reduced risk of creating a dead zone in which molten solid can settle to solidify. Preferably, the sidewalls of the second section lie along planes which diverge outwardly away from the valve member to avoid a restriction of the flow of molten solid and to discourage any bridging which might otherwise lead to a solidification of the molten solid. Furthermore, in the known outlets the valve member and the seating member have conical surfaces which mate to form a seal. In use the valve member tends to stick leading to the need for large forces to rotate it, which leads to wear. However, in accordance with the present invention, the surfaces are convex in a plane including the axis of rotation of the valve member. This prevents the jamming experienced with conical members reducing the force required to rotate the valve member and generally decreasing wear and thus increasing robustness. Also, if the sealing surfaces are hemispherical the same benefits accrue and the valve assembly can tolerate slight misalignments of the refractory parts. This will be of considerable benefit in industries where unskilled labour is employed, e.g. in the steel and iron industries.
In accordance with an alternative or additional aspect of the invention, the outlet has a branched passage which has a plurality of outlet ends spaced from each other so as to be aligned with the second section of the wall in different rotational positions of the valve member, the inlet ends communicating with a common outlet end of the passage. If, in use the valve member is always rotated in the same direction, both to open and close the passage, each of the inlet ends will be used in turn, so reducing the wear and tear on each and increasing the life of the outlet.
The dimensions of the sections depend on the number of inlets and whether they are to be used singly or together.. Where there is a single inlet and a single second section, the first section preferably comprises from about 19% to about 81% of the length of the wall. Where there are two inlets and two second sections the first section preferably comprises from about 38% to about 62% of the length of the wall. Where there are two inlets and a single second section so that the inlets are used individually preferably the first section comprises from about 69% to about 81% of the length of the wall. The number of inlets may exceed two. The dimensions given are preferred in the case of casting articles of steel in a foundry, and other values may be appropriate when the molten solid is different, e.g. or another metal such as iron, or steel when processed in a steelmill or a
A suitable material from which to form an outlet intended for a metallurgical vessel, is graphitised alumina and the valve member and seating member are preferably made by isostatic pressing. Although a preferred embodiment of the invention is utilised in a vessel for molten metal, the invention can be applied to a vessel for any molten solid which is liable to freeze blocking the outlet.
It is a much preferred feature of the invention that the flow of molten metal emerging from the down pipe be coherent, i.e. having substantially parallel sides, irrespective of the extent of opening of the inlet. To ensure this, in a further feature of the invention, means to control the shape of the stream are present in the passage. Most preferably the means comprises elongate generally parallel straight sided ribs, radially spaced about the passage. Preferably at least two such ribs are present.
The invention also extends to a vessel for molten solids having a floor having an opening containing an outlet as defined in this invention, the top of the second section of the wall of the outlet being approximately level with that of the surface of the floor inside the vessel.